This invention relates to electroconductive coating compositions and to coated articles employing them.
In the manufacture of many articles employing nonconductive substrates as a component part of such articles, it is often desirable to reduce the surface electrical resistivity of the nonconductive substrate to a value below 10.sup.14 ohms at 10 percent relative humidity. For example, paper which has been rendered electroconductive may be used to distribute electrical stresses in various insulating products. Also, the support normally used in electrographic, electrophotographic, electrostatic and other nonimpact printing processes is conductive or is coated with a conductive layer, and as such plays a role in the formation of the image or the record.
In electrophotographic recording elements, a photoconductive layer stands in contact with an electroconductive layer or sheet, the latter being present to enable charging of the photoconductive layer and to carry off the electrostatic charges from the areas of the photoconductive layer undergoing an exposure to light rays. As the photoconductive or photoresponsive material, there is presently being used a specially treated zinc oxide coating. Where light strikes portions of the paper treated with such a light sensitive material, the electrical charge is dissipated in those areas exposed to light. As a result, there is left a pattern of charged and uncharged areas. The charged area will then be effective to attract an oppositely charged powder, or other usually particulated image forming material. Such a powder will not be attracted to the light affected discharge areas, and the powder may thus be deposited on the paper in a pattern which corresponds with the charged areas. Generally, such an image forming material may then be fused, or otherwise treated, on the paper to make the image permanent. Such an image forming material is often a wax coated finely divided carbon black which will fuse when heated on the paper.
While other nonimpact printing processes differ from the above in that the images are created by electrical dissipation of the static charge in nonimage areas, all require as a common characteristic an electrically conductive substrate such as an electroconductive paper.
Conventionally, paper and other nonconductive substrates have been rendered electroconductive by coating same with various electroconductive additives such as water-soluble quaternary ammonium polymers as ememplified in Silvernail et al, U.S. Pat. No. 3,011,918, Booth et al, U.S. Pat. No. 3,544,318, Cavagna et al, U.S. Pat. No. 3,479,215, and Rogers et al U.S. Pat. No. 3,320,317.
Because of their relatively high cost electro-conductive resins are seldomly applied directly to raw paper stock. Instead, the base substrate, such as raw paper stock, is generally precoated with conventional paper coating for impact printing such as a combination of binder and pigment. Subsequently, the electroconductive resin is formulated in an aqueous coating composition containing in addition to it a film-forming binder such as starch, polyvinyl alcohol, or a latex and a suitable pigment such as clay, calcium carbonate and the like. The type and amount of binder has a pronounced effect not only on the surface electrical resistivity of the resultant layer but also upon the resistance to organic solvents used in subsequent processing. As would be expected the solvent resistance (holdout) of the electroconductive coating to the aromatic solvents generally used in the photosensitive coating and to the aliphatic solvents generally used in liquid toning is very important.
In such conventional electroconductive coating compositions, it has been found that very few binders provide the degree of solvent resistance required without deleteriously affecting the high conductivity required in the electroconductive layer. Furthermore, compositions used heretofore require the additional step of precoating the raw paper stock if they are to provide useful electroconductive paper compositions.
Therefore, it would be highly desirable to provide an electroconductive coating composition which can be applied directly to an uncoated nonconductive substrate to produce an electroconductive substrate having excellent resistance to solvents such as the aromatic and aliphatic hydrocarbon solvents.